Technical Field
The present disclosure relates generally to wireless communication systems, and more specifically to techniques for reducing spurious signal emissions.
Related Art
In a Long Term Evolution (LTE) communication network, uplink radio resources may be allocated dynamically to user equipment (UE) based on different application, data rates, and quality of service (QoS) requirements. Specifically, a UE in an LTE communication network may be allocated varying numbers of resource blocks (RBs) for uplink transmissions from the UE. Each RB may extend over one slot period (e.g., 0.5 milliseconds (ms)) in the time domain while occupying a certain range (e.g., 180 kilohertz (kHz)) in the frequency domain. The RB allocation for a UE may vary in both bandwidth and frequency range between consecutive LTE sub-frames (i.e., two 0.5 ms slot periods). Based on the RB allocation, the UE may transmit data at the center frequency of the allocated RBs.
A conventional transmit path may include a frequency mixer that modulates a carrier signal from a local oscillator (LO) with a modulating signal from a baseband filter (BBF) representing the data to be transmitted. The carrier signal may have a fixed frequency that is relatively high. By contrast, the frequency of the modulating signal may be at the baseband (i.e., near zero hertz). Moreover, the frequency of the modulating signal may be varied in order to achieve an output signal at the center frequency of the allocated RBs. Up-converting the modulating signal from the baseband to the frequency of the carrier signal may give rise to spurious signal emissions including for example, but not limited to, residual sideband (RSB) signals and intermodulation (IM) products (e.g., primary and second 4FMOD signals).
In particular, when RB allocation is narrow (e.g., one RB), the operation of a conventional transmit path may generate SEMs that fall outside of the frequency band allocated to a UE and interfere with transmissions on other channels. A UE may adopt additional maximum power reduction (AMPR) parameters in order to suppress spurious signal emissions to conform to 3rd Generation Partnership Project (3GPP) LTE specifications. However, adoption of AMPR parameters may reduce the output power of the UE and thus limit the UE's coverage area.